Semiconductor lasers are increasingly being used in commercial applications. However, these diodes generally have the problem of a broad spectral linewidth and in many applications, such as in the communications field, it is necessary to have a laser with a higher spectral purity, as well as a laser having a stabilized and controlled oscillation center frequency. This growing need has stimulated a plethora of semiconductor laser frequency stabilization techniques.
One broad class of frequency stabilization techniques utilizes some form of optical feedback. In this technique, a predetermined fraction of the laser's output power is returned to the laser from an optical element, such as a single external reflector, a grating, a thin glass plate, a fiber Rayleigh scattering device, and a phase conjugate mirror. (See e.g., F. Favre and D. LeGuen IEEE J. Quantum Electron. QE-21 1937 (1985) and references herein.) Vahala et al, 49 Applied Physics Letters 1563 (1986). The extreme limit of the optical feedback method is the extended cavity or the "external cavity laser." In this method, the semiconductor gain medium is located inside a larger optical resonator. The external cavity laser has spectral characteristics that can be quite good and these characteristics are often comparable to other laser systems, such as dye and gas laser systems.
Other known methods to obtain narrow linewidths and frequency control include optical injection locking using a stable master oscillator as the source or a second type of approach utilizing electronic-servocontrol to stabilize the diode laser frequency to a separate frequency reference. Such a separate frequency reference can be obtained from a resonant cavity or an atomic resonance device.
A third approach for diode laser frequency stabilization includes a hybrid method that utilizes electronic and optical feedback, the latter coming from a fiber optic cavity. (See e.g. Favre et al, 21 IEEE J. Quantum Electron 1937 (1985). In addition, some recent studies are reporting that it is possible to stabilize semiconductor laser frequency by establishing feedback from a monolithic coupled grating system. However, because of the intrinsic spectral characteristics of semiconductor laser frequency noise, the only systems which have been found to achieve substantial linewidth reduction are those which incorporate some form of optical feedback, or which use very fast electronic servos (typically requiring servo-bandwidths of 20 MHz or greater).
Standard designs of semiconductor lasers have very short optical cavities (typically a few hundred microns) and have relatively low reflectivity mirrors. The resulting laser resonator has a low Q (quality factor). For a number of reasons that are related to the physics of the interaction between the optical field and the carriers in the semiconductor, the semiconductor lasers also show excess frequency noise. When these two factors are combined, they result in diode lasers having relatively broad spectral widths, the narrowest of which are on the order of tens of megahertz wide. Thus, the conventional semiconductor lasers are unacceptable for many applications because of their broad spectral widths and the instabilities in their oscillation frequency. Furthermore, the oscillation frequency of a free running semiconductor laser is strongly dependent on the injection current, the operating temperature and the optical feedback.
There are numerous United States patents which disclose optical feedback in semiconductor lasers. Many of these use a Fabry-Perot resonator, such as the one disclosed in the Kaminow Patent 4,198,115, incorporated herein by reference. The following patents are typical: Hadley et al, No. 4,751,705 (optical injection in a single end-element of a semiconductor laser array to lock the array output); Smith et al No. 4,556,980 (injection locked semiconductor laser using a gas laser produces a narrow linewidth and frequency control); Haus et al No. 4,464,759 (semiconductor diode laser with microwave mode locking); Dutcher et al No. 4,752,931 (injection seeded Q switch laser with selectively introduced light from a master oscillator); Fujita et al No. 4,677,630 (frequency stabilization technique utilizing optical feedback of a fraction of the laser output); Beene et al No. 4,606,031 (electro-optical feedback to a transducer to change the resonant characteristics within the laser cavity); Smith et al No. 4,221,472 (optical and electronic feedback utilizing a Fabry-Perot interferometer to adjust cavity length); Liu No. 4,181,899 (a glass laser utilizing an opto-electric Fabry-Perot resonator located within the laser cavity to tune the laser); and Byer No. 4,455,657 (optical injection locking to stabilize a high gain laser).
The foregoing references illustrate current approaches to the solution of laser stabilization. However, none of the approaches in the disclosed references have the performance with respect to linewidth and frequency stability that the present invention provides. Unfortunately, the frequency modulation capabilities are usually sacrificed when the laser linewidths are reduced by optical feedback. With most of the frequency stabilization techniques, there has been found to be a trade-off between the size of the diode laser's linewidth and its FM capabilities. Usually, a one-to-one correspondence has existed between these quantities such that if the laser linewidth is reduced by a factor of ten, its FM sensitivity to injection current, (typically on the order of 3 GHz/ma) is also reduced by roughly the same factor of ten.
The theory for optical feedback in lasers is still being developed. An early publication that describes one possibly acceptable explanation, coauthored by the inventors of the present invention, is Hjelme et al, "Novel Optical Frequency Stabilization of Semiconductor Lasers," Topical Meeting on Semiconductor Lasers, Feb. 10-11, 1987, Albuquerque by Optical Society of America, incorporated herein by reference. This article explains the theory with reference to a diagram of a conventional optical feedback arrangement using a Fabry-Perot confocal cavity set up in a conventional "FIG. 8" arrangement to lock the laser frequency. Although this diagram is very similar to FIG. 8 herein, the article presents a conventional approach to explain the theory being proposed and is nonenabling as to the present invention because of some very critical omissions. Nevertheless, the article is helpful to provide the theoretical background necessary to fully appreciate the present invention.